combined two wordpress themes from same provider

hi, I have two wordpress themes using two particular function, or plugin. one is for job board posting and the other one is for ask/answer activity. the main emphasis is the job board posting, and it will be completed with the ask/answer activty. both are themes for wordpress, so they both have their login system, separated. I need to have on single login for the whole website, and add as an option for the users a facebook connect. both dashboard has to be regrouped for ergonomy in on single place (but not to messy though). the front page uses the job board's. right column should have the login module, and a module with "latest questions". top bar will ease the navigation from the "two theme" for example: about us / blog / job board / Q&A. at the end it should look like one single site. I can do some css, and language translation as well (the website is not in english). thank you