We do it all the time. I don't think we've done it at signatures ~ the portion sizes are more realistic. (And yummy, I want to taste two entrees, not share!) We have shared appetizers and desserts at the finer establishments. We have never been charged a "plate fee" but it is possible. Often the food is split in the kitchen and plated for each of us. It is your money, eat what you want. You might consider upping your tip a bit to compensate for the extra work with smaller total bill.
ETA ~ Just remembered ~ we went to Cali Grill and shared a cheese plate, a sushi platter, a bottle of wine and each had dessert.