Did I mention that you could do this for free?
Once you've got your head around the possibilities, here's some great fun double-team options you can play, I've selected all these options as they are great fun in their own and can be reached within a couple of hours of play time for free as they are all low tier options (or very cheap to purchase).
SU-26: The Tinpot Tank - Tier 3
The SU-26 is tier three, meaning you are two steps (SU-18 to SU-26) from the bottom tier. This is arguably one of the funniest tanks in WoT's, aritllery with a turret is unique and makes this tank a joy to play. Maxed out with the best equipment, a load-rammer to speed up your Rate of Fire and some camoflague (and XP), this tank is a beaut. It's exceptionally accurate, with a high rate of fire. It can even 'put the willies' up level seven tanks with sustained fire.
Strength. A team of two SU-26s can be devastating. Think of the SU-26 as an infantry mortar team, moving in 100m behind the front line. It can suppress a charge, or zero-in on a 'hiding' opponent and force them out. A combination of 2 mediums, a smart fast tank some accurate SU-26's can completely own a flank. Ripping 10-15% off a KV per shot, double trouble for anything in tier 4-7 and instant killers in tier 2-3.
Weakness. It's actually got good options (thanks to its turret) against most scout tanks that come after it. Sadly the T-50 has put the SU-26 in a tricky situation, as the T-50 can hit your base as the SU-26 barely leaves it! That said, clever placement and map knowledge means you're barely suckered, but watch out for Ensk and Prokovia. Staying in behind a big group of tanks helps, and they'll love your accurate mortar work.
T-50 Light tank - Greased Lightning - Tier 4
If any tank is due a significant nerf, it's this one. Put simply, this is the best Tier 3-5 tank across the board. Nothing has it's acceleration, bounces shots, goes up hills, circles like a buzzard like a T-50. This is one tank that has the whole enemy army reacting to it and usually firing off everything they have. Taking two T-50's is brilliant fun, and twice the headache for the enemy. Level 4, but everyone should own one.
Strength. Really quick. Adding the cheap suspension boost means this thing barely slows for hills, it's always a useful tank. In higher tier games a pure and menancing scout, in lower tier it's an awesome mixed light-medium with a solid gun and great firepower. It's speed, movement and 'kinkiness' make it the bane of enemy loners as you can simply run round them. Expect a lot of love (bullets) in this tank. Double-teaming makes no effect on your tier and double the fun, after you die, you can watch and advise your maniac buddy!
Weakness. It's got good bounce on very light armour, sporting down a flank will often get you one-shotted and in larger games those big tanks have fast traverse - taking you to one shot deathville. Gotta hit you first.
T-57: Agressive Artillery - Tier 2
The T-57 is a weird level 2 arty. It's got great armour for it's tier, good speed and movement and actually can beat off a lot of tanks. Double teamed at high tier, you become a standard arty (although end game base attacks become possible). In lower tier, this tank works very well as a tank hunter, giving you some serious prestige in close range killing.
Strength. Quick, well armoured, bounces shots and with an above tier damage rating. This tank can bring the pain and is cheap to rebuild.
Weakness. Against anything serious - you are toast.
Panzer 4/T-34 combo - Tier 5
Taking either both or one of each will guarantee a fun game. The Panzer 4 has an awesome top gun, giving you above tier reach to hurt mediums and heavies. The turret is well armoured, but the chassis isn't, meaning hull down is a must. The T-34 is a classic design, and on reputation should be dominant. If you know the field and other tanks well, this little fighter can really harass. It can penetrate a Tiger from the front and offer 20-30 shots a minute.
The Panzer has the raw punch and stopping power, but lacks the speed and low profile to spot. It's also cumbersome at close range as the turret traverse lacks. The T-34 fits well with it, offering a more balanced approach with a solid, high ROF gun.
A tasty combination.
PanzerKampfWagon B2 (actually a french B1 Char)
The PzB2 costs £4.22, and for that you get a lot of value for money. Thanks to the tiering restructure, the B2 habitually starts game at 'top of tier', and at only tier 4, the worst you can face ain't that scary. In line with other premium tanks, the B2 is cheaper to repair and makes more money than other 'standard' tanks. This means that the B2 can quickly churn you up some ready cash. Unlike most other premium tanks, this one's actually a joy to play.
With front and side armour and 60, the B2 can ignore a good number of tanks in battle and concentrate fire on the heaviest armed (the M3 Lee, Hetzer, PzIII and SU85B are its natural enemies. It's gun is sub-par, but perfectly capable to punching holes in the majority of tier 2-4 tanks. Unlike it's appearances, the B2 is surprisingly agile and has great movement, although the top speed is capped at 30kph. This means that you are a hard target for artillery and not a heavy that can be jumped and circled by a leopard or t-50 that easily... you are not easy prey like a lone KV for example.
Taken as a pair, the B2 is hilarious. My buddy and I have jointly sandwiched a fixed gun STuG III between us and then shot his side armour to bits, I've run a whole game ramming opponents and drawing fire and with the great movement, its a natural city fighter. You need to be tactically aware in a B2 though, as you can't "race back to base" if you've been flanked ... so keep your buddies in check.
Best Combination Ever: B2 plus Su-26.
One of you plays enormous bullet magnet, while the other one racks up the kills with a super accurate mortar. Great fun and 'productive'.