I have been peddling a completed middle grade novel for some time now and have had three requests for full submissions. The first two came back with personal rejections and invitations to submit future projects. I am still waiting to hear back on the third full but have been told to expect a wait of 3-5 months. Having endured an additional 40 query rejections and several others on partials with seemingly personal invitations to query future projects--I now have all my proverbial eggs in this one last basket.
However, I am nearly finished with my latest "future project" and wondered if I should wait the 3-5 months to hear back regarding the full sub of my last book before sending queries to the inviting agents on the new one. Is it unwise to overlap the query/submission process of two different projects?
Their slow pokieness should have no bearing on your forward motion.
You can have a variety of things in submission at any given time.
IF Slow Poke Publisher makes you an offer, great. If they pass, you've got other irons in the fire.
If EVERYONE makes you an offer you'll have fun juggling offers.
There's no down side to getting your "new" work out into circulation.