You can be a member of multiple programs, but you can only have one card attached to one program at one time. Trust me, they've thought ahead and put restrictions into place so you can't double/triple/quadruple (etc) dip on rewards. If your credit card is already attached to Delta and you now want to focus on getting United miles with your dines, simply add that credit card your United Dining profile and it will automatically drop off the Delta profile.
If you do any sort of client entertaining and you have an option of what restaurant you go to, you should browse the restaurants near you that participate in rewards programs. You also earn the points from your credit card spend, so if you used a British Airways Visa on a $1,000 dinner at a restaurant that participated in the United dining program, you'd earn:
1,000 x 1.25 normal earning ratio on BA Visa= 1,250 British Airways miles
1,000 x 5 for VIP dining member (remember, all you need to do is "dine" 10 times to get VIP, which can be as easy as buying 10 sodas from a participating restaurant and having them ring them up separately)= 5,000 United miles
1,000 x 10 for the May/June bonus = 10,000
= 16, 250 miles!
