I used one example. An example where DD2 tends to wait/do something else. You are extrapolating from this that this is always the case because DD1 tends toward the dramatic and noisy. I assume that you are doing this because you have decided that UP means 'giving in to whoever is noisiest'. Hardly rational.
In the example squeakytoy gave, I would start by offering a picnic as an option. So both children want to do something different (and expensive!). If I could afford it, we'd do one on saturday and one on sunday, sounds fun to me too, I like the zoo and theme parks too and can see why they'd prefer those options to a picnic. I know at 5 I would have done too. I would explain that both places cost a lot to get in to and that they would probably wipe out our going places budget for the month though.
If we couldn't afford both, I would explain that and suggest ways round it. Off the top of my head, we might decide together on which one to do this month and then decide on a date for the other the following month. DD1 likes having things to look forward too! DD2 would accept 'we're going soon', being 2 she doesn't have the same concept of time.
If we had other plans, say we were meeting someone for a picnic, they would know in advance that we had planned to meet a friend, they would be looking forward to that. If we needed to drum up enthusiasm, we'd plan our bigger days out for other weekends and then get them choosing their favourite things to eat on our picnic, planning which toys they were taking etc.
Off out now, to play in MIL and FIL's garden with the sprinkler on!