Sorry, it appears I’m blind tonight. Of course Post Object allows more than one selection. I’ve tried this and I’m not seeing the same issue.
My guess is that you have a filter or an action somewhere that is causing an infinite loop. Do you have an acf/save post filter on the site? If you do comment out the add_action() lines and see if that clears up the problem.
There was a similar issue posted here http://support.advancedcustomfields.com/forums/topic/update-value-function-causes-timeout/
