I'd sit down with him and ask him to tell you what to expect. Timing of communications to the other attorney, a minimum 24 hours to return your phone calls. I'd say telling you that you aren't his only client is HUGE no-no in my book. I personally might start letting some of payments slip behind and tell him "you're not my only bill and I'll pay in plenty of time".
But seriously, I think you need to make an appt and talk to him. Ask him if you should seek out another attorney if he's got too much to handle right now. You don't want this to hurt you or his office. Ask him what you should do. See what he says. I wouldn't bring up the 2nd opinion lawyer. It's not necessary to be rude or disrespectful. Just state that you are concerned and ask him what he suggests you do or tell you what to expect. Divorces can take a really long time. I think if he could help set expectations with you that would help. Hang in there.