A couple updates from testing today. I know people have said they weren't able to do this, but I have tried before and today was again able to book a second Tier 1 after only using the first Tier 1 and being past my windows for the two Tier 2's (I did not use either of them). Here are the two Tier 2's I had booked:
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I also had Living With the Land from around 1:30-2:30 (Tier 1). After I used LWTL (first ride I did), I went in and tried to book another Tier 1 while past the Tier 2 windows but without using or tapping them. The first screen looked like it wouldn't let me do it:
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But when I selected one of the times shown for LWTL again (only remaining Tier 1 at that point), it went right through.
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I did notice that once I went back into my FPP's on the home screen that the two Tier 2's had disappeared at that point and all that showed was the remaining Tier 1. I tested the FPP (just tapped and it turned green at LWTL) and then was able to book yet another. (This time I just cancelled it after).