Currently two agents have requested partials on a cross-cultural YA Coming of Age story.
I also have a quite different novel—about narcs in high school-- in the polishing stage right now which I can query at the end of the month.
I understand agents are busy, and that they may reject and never even get back to me. I don’t mind that. but how much time should I give the partials before I start querying the second novel?
The partial requests are 2 weeks old on the first, and about a week on the second.
At this stage of your career you have to press ahead on YOUR timetable, not an agent's. If you wait for agents, you'll be writing geezer lit not YA by the time you hear back.
If your novel is ready to go, send it out.
What you need to be careful of is sending the second novel to people considering your first. Don't do that.
If an agent has passed on your first novel, you can query with the second, but if the agent has not, leave them off the submission list for novel #2.
And just a bit of inside info here: all those agents and agencies that say "if you haven't heard from us in 30 days, assume it's a pass" are nowhere NEAR that caught up most of the time. I'd give those folks at least 90 days if not 120. I know for an ironclad fact that some of my colleagues are still working through queries from September (myself included, although I'm more caught up now than I was yesterday!)