I don't use RMU, but I use something similar to APs and it's 'one at a time'.
I don't use initiative either. At AR1, those who have completed their action (1 AP action) act. Then at AR2, those who have completed their action (2 AP action, or 1 AP action started at AR1) act. And so on. So if you want to perform a 1 AP, a 2 AP and another 1 AP action, you act on ARs 1, 3 and 4.
If two people act in the same AR, actions are simultaneous.