Well Im a real newbie... Ive only hatched one time and just recently. But from everything I've seen on here.. I think staggering a hatch is something to be avoided, at least I would. Some I think with tons of experience, or a seperate hatcher, do well with staggering, but it doesent sound like most do.
If I were you, I'd stick my present batch of eggs in the 'bator and then send the seller of the others an email. I'd say that since there was such a delay in initial communication from them... even just letting you know the ETA... that you really do require they hold off on the shipment and send them to you in 3 weeks... or give you a refund. Explain the circumstances.
I'm really NOT for taking back a payment or posting negative feedback in almost any circumstance... but really? They make you wait that long for an ETA and ignore your inquiries? They should at least be willing to send your batch in 3 weeks. One of the nice ladies on here says thats not too long to combine chicks together in a brooder, so it should work. If they wont do it for whatever reason, then you should ask for a refund so you can buy another batch of the kind you wanted in 3 weeks from someone else. If they wont do that then I would mention speaking to Paypal.... but I would probably never do it, myself LOL!
But if you decide to go ahead and accept them, I'm sure some others with experience staggering hatches on here will help you!
I'll only mention what I keep parroting on here that I read on some university website, that might help you with lockdown
If you're hand turning.. otherwise just ignore this LOL.. Supposedly, turning the eggs after day 14 is no longer
strictly necessary (their words).. so you
should be able to stop turning the second set of eggs when you lockdown the first (if they aren't more than 4 days apart), so you're not upsetting the temperature and humidity.
Hope all goes well for you
Edited to say: Ooopsie! Only just saw your reply!