USAC has a guest riding provision, you will find that it can be a PIA when using an online service to get your guest team listed. Getting results correctly credited is also a PIA.
If the teams are geographically seperate racing for two teams is a bit cleaner, trying to race with two teams in the same area is going to create a lot of issues. Teammates are going to question effort, loyalty and why you get two schwag list, sponsors are going to be unhappy if you are using other stuff, and competitors arent going to cool with having to go into each race trying to figure out who you are worling with.
Ive been on two teams the last couple years, 400 miles apart.