I've done 2 jobs for the same client this weekend. Do I put them on the same invoice or send him two separate ones?
I suggest you simply ask your client what is his/her preference.
For some of them, it's easier to manage separate invoices and taxes if the projects are done for different brands. If the projects are done for the same brand than it's usually less of an issue and it can be on one invoice.
One thing clients don't like about multiple invoices: they need to make multiple payments... or calculate themselves the total amount. It can be annoying for them. Sometimes it can even be used as an excuse on why they forgot to pay one of the invoice.
What you can do is simply send you 2 projects on the same invoice and mention to your client that you can send 2 invoices instead if he/she prefers it. That's the fast way to do it!
If I may suggest a good online service to use for your invoicing and projects' management, have a look at Freshbooks. It's really easy to separate your projects' invoices with this and clearly show to your clients how your time was spent for each projects, even if you send 2 different projects on one invoice.