You look at the business goals and based on your experience with each platform decide where the budget will likely yield the best results.
Let’s say, for example, your goal is lead generation, and your platforms are FB Ads and Twitter Ads. It’s pretty obvious you’re going to get a lot more from FB and the lion’s share of your budget should go there.
Other considerations would be historical performance, your management fee structure, and simply budgeting enough for a platform to allow for practical optimization.
Once you get rolling you’ll see some real performance figures and can adjust accordingly.