There are a variety of ways of doing this.
However you do it, it needs to be documented and you need to have an agreement of some sorts.
It can be done as a partnership which means that the partners have financial liability but, as you are putting different amounts of money in, you would need a partnership agreement stating exactly who does what and who puts what in and who gets what out.
In a partnership, you can do whatever you like you can have different riding styles, BMI Internet, BMI property, BMI computers, BMI motors etc.
Financially, they would all come under the same banner.
You can set it up as a limited company and you would then need direct service agreements/employment contracts which do what employment contracts normally do and the shareholders agreement stating who can sell what shares to who.
Each limited company would be able to do whatever it was able to do under the terms of its Memorandum & Articles. That could be everything that you want to do in the business or it could be narrow. Hence, you could have several limited companies all owned by you both each doing something different. You could have one limited company owned by you both and then a whole load of other limited companies underneath that owned by the one company which is subsequently owned by both of you.
Until you get bigger, easiest way is probably to set up one limited company with directors service agreements and shareholders agreements and then, when you want to branch out set up another limited company to deal with a completely different type of business.
Probably the most important thing is the agreement that you have with regard to what each of you are going to put in, what each of you are going to do, and what each of you are going to get out.
Can I clarify anything for you?