Two separate sites, shared user control?

Hi! i have currently a WP based servicesite with over hundred users.

Now im building a new service with wp.

Both are under own domains on hostings service.

Is there any way i can connect these two so that user registers to other, they also can log with the same credentials to the other site.

Also i would like to move the current users to new site also when taking the new site in use.

Im in a hurry, so all the help is much preciated ?? Thx to all in advance!

-Tim