Both of my children went through a similar stage. Usually, they would just climb in our bed to tell us that they had a nightmare/bad dream. Since they were scared and seemed to need comfort, we allowed it.
Granted, if our children had their way, they would be sleeping with my husband and I every night (and they're 9/6 now). So, we learned to draw the line to just the nightmares. Also, a lot of evenings, my husband would carry them back to their bed and lie down next to them until they fell asleep. That really helped ease some of the burden. Maybe you and your husband could alternate? I tried to reinforce that "Dad would protect them" which seemed to help when they kept asking for "Mommy." Ultimately, they didn't want to sleep alone so sleeping w/Dad was still better than sleeping by themselves. Plus, I think, putting them back in their own bed helps to establish ground rules of where they sleep.
Eventually, the 2 children could share a room (which helps alleviate some fear of being alone). We moved my younger daughter in with my son when she was 3.5-4 yrs old and it pretty much stopped (again, except for really bad nightmares).
Good luck and hope what I mentioned helps :)