It's not to complicated and so far I have had some great success with it.
1. You start with marauders, get stim for them.
2. Get stim for marines
3. get plus 1
4. You save up for maraiders (Occasionally you can afford plus 2 depending on your feeds.)
5. After you get maraiders you are purely focusing on getting the shrapnel grenades.
6. Once you have afforded that you'll get one or two more upgrades
7. Save up for two overseers
8. get about 8 weapons upgrades then increase merc health.
9. Start getting overseer upgrades

Overall you'll being playing a bit more aggressive, trying to be in the middle for the healing as much as possible. You'll have a lot of mobility so trying to spread out and avoid being choked as much as possible.
The toughest part of the game will be while you are trying to get the shrapnel upgrades since other compositions will be strengthening up while you are trying to really take advantage of the aoe.
Once you have the overseers it'll be difficult for other teams to really engage you. You'll start having creep spread (Extremely useful if your teammate has a slow moving hero like the thor.)
You'll be utilizing changlings to help take any engagements and later in the game to help you to snipe or pressure hero's.
The changlings will be a serious aid versus melee compositions as your opponent will not be able to commit a large amount of forces.
Maraider and brutalisks splash can hit multiple changelings and also the brutalisks can tank 3-4 really early so they'll be a bit more challenging.
Blink stalkers are susceptible to changlings since unlike brutalisks and have low health pool. The trade off is you'll have trouble cornering them and they'll be the toughest for you to engage.
Roaches can be tough to face as well.
Phoenix and banshee's can have their challenge. You'll be able to almost always avoid the banshees opening salvo though.
Other than that, you'll steam roll any melee composition like ling marine, warp lots with honey badger ect (The temple zealot even without the changlings you'll be able to kite very efficently.)
Overall the best partner is one that concentrates on a hero early and ups to something like the Goliath (or possibly and air merc)
If you are playing that style you'll need a lot of space, you'll mostly doing your own thing needing a fair amount of room to split your units, kite or set up a concave/sorround. If your partner keeps his units around you trapping and limiting your mobility you'll be suffering.
Good options for heros are the Thor, Pheonix, colossus or honeybadger.

The benefit of this style is you'll be able to concentrate a little bit earlier on marine upgrades once you have a solid lead with your mercs. You won't really have true hero like the archon, dt or spectar until solidly in the lategame. The plus is the overseers will help you snipe any of the enemies heros, and if your ally can aid in it with phoenix it can be quite the feed.
Overall I have found that this style works extremely well even if you fall behind in income.

On a side not, how do you get an avatar? Am I just blind and not seeing it in the settings?

only premium members can get avatars

By premium members he means donors. You can donate now 10$ or more and get avatar+skins.

Apocalypse

Settings > Profile > Edit Avatar

Thank you, I am blind and totally missed that the options tab extends once you select profile.

he was talking about forum avatar :c

Welcome to the forums Apocalypse

I know . Was trying to get josin a easy 10$ :P