While looking for the representation for my first novel I've started working on the second book of what I plan to be a trilogy. What if I never find an agent for the first book and the second book turns out to be better than the first? It firmly stands on the shoulders of the first one and publishing the second without the first wouldn't make much sense.
Should I shelf the trilogy till/if I find the agent interested in my first book?
Dear dog, you do like to worry about things you have no control over don't you?
I let my lawyer worry about "what if" and I worry about what's on my desk right now. What it costs in cash (even at Shyster Snark's discount rates) it saves in peace of mind.
Keep writing. Worry about this later, if at all. If you REALLY need to worry about something you have no control over: killer bees.