well the 7070 has a single slot for a peace dollar but others include both. In the end the choice is really yours
The NGC registry has two Peace Dollar slots. But @green18 is correct. "You're the captain of your ship."
My Dansco has just the one spot (as mentioned by @mrweaseluv ) so I just have the one coin. The 1921 has become more expensive ever since the uptick in 2021 (in part due to the 100th anniversary promotion) so if you are on a tighter budget, I would go with a nice UNC common date (1923 for example). However, the 1921 is a cool coin and if you can afford it, then go for both types.
That's the beauty of collecting..... its totally up to you. For me and my budget I'd do a nice low relief and call it a day, but that's me. Look at the post above this one, @dwhiz has an amazing set that are all nicely circulated. The options are endless and there's no wrong answers.