Two females in the small tubs? I have done it an had a problem. The problem was the second female dropped five days after the first. Why? Because I only had one male. The problem was baby stealing which led to fighting an I lost eight pinkies. Plus the smaller ones got beat an dominated by the older pups. I always use two males now. A rat Norway by its self for a month causes problems, so instead of one in a tub it’s two.

if you run one male stagger your breeding. You could run four females an leave one in with the male 17 days. That way you always have two together.

If you build a rack with the large tubs you could get away with almost anything except leaving the male in. While you could leave him in the outcome would suck for litter size. If you use the large tubs run a 1x1 across the inside 4x4(2x2 if you used). That way the bottom of the tub will be supported. As the rails alone might (won’t) not hold. Be there an done that.

Read around here an some Rat fancier sites. You could do fine with rotating one male but you’ll need a add things to the set ups. Adding a sixth tub to the rack might fix any problems. Cycle the male an separate the females at 18-19 days.
One male with two females will cause the drop days to by off. It takes a few days sometimes by bring the female into cycle an if one male spends more time with one you could have a two week difference. My glass tank days showed that. Same as having one boar in with six sows. Better to bring him one at a time. An giving him a break or he’ll drop dead from wearing himself out. Has to be seen to be believed (an I saw it).

Good luck!