Hey Sergio,
Let me address the LLC question (more my lane). With any partnership, be it business, real estate, whatever, make sure your operating agreement (the governing document of an LLC) states a few key things.
1. Expectation of the members (owners of the LLC).
Since you both have your own portfolios it's important that you are both aligned in the expectation of what the priority is. A good example of this would be if you (or your partner) found another property you wanted to snatch up. As an owner of the LLC you have a 'duty of loyalty' to the company that basically says you have to place the LLC's interests over you personal ones. In this case, you'd have to bring the property to your partner to see if it's something he would like to be involved in. If he doesn't - all cool! Go ahead and snag it. The expectation should be stated, though.
One way to do it is to have a firm understanding of what the 'business purpose is'. If you are going to be buying up apartment complexes with this LLC and you are going to be building your own single-family portfolio, then there wouldn't technically be a conflict of interest since the portfolios are different. Does that make sense? The long and short of it is transparency.
2. How you guys can break up.
It's so important to have an 'out' clause in your operating agreement. Without an 'out' clause it's very, very hard to get through to an amicable separation if things turn to sour grapes. This should include how to value each member's equity in the LLC, how that is calculated, how the separation of portfolio assets will go, among a whole host of other things (including what happens if you or your partner becomes disabled, divorced or passes away).
3. Compensation
Everything has to go through the LLC, right? But how does that break down in terms of the distribution of profits? Are you going to reinvest profits into the LLC until you reach a certain multiple on your money (and at that time distribute percentages of profits)? Are you both bringing the money? If so, how much, respectively? How are taxes going to be handled? All of these questions can weigh in regarding how you are legally (as well as morally) capable of being compensated.
Just some input regarding the structure of your LLC. All, of course, being educational in nature and not legal advice!
Best of luck!
*Goes without saying, but this answer doesn't create an attorney-client relationship. Further it is not and shall not be construed as legal advice.