You are correct, no more than one preapproval or special offer may be sent by one host to any one guest at a time. I noticed this problem quite a long time ago, and I wish there were an update allowing for multiple preapprovals/special offers from a host to the same guest because there are many situations where it is needed. As in your case, where the guest will be coming many times, and wants to book all the trips in advance. Or when a host has two listings, say two different rooms available in their home, and the guest wants to book a stay in one and then book a stay in the other. THese things cannot be done, because as you found, after you have sent one special offer, if you try to send another one, the new price for the 2nd offer becomes the price for the first offer. So the two offers get conflated in the system.
So the only way to do it, if you will be having a guest do more than one reservation, is that you send them one offer at a time. As soon as they accept the first offer, right away you can send them the 2nd offer. Guests can book any number of reservations with you (even ones whose dates overlap if needed, eg if they have additional familiy members coming and need to book an extra listing for more space) but you have to do it one at a time. I did this with a guest who wanted to do 3 separate reservations with me. I would send her the first offer, she would accept and email me she was ready for me to send the 2nd one, and I then sent that right away, and so on.
