There is no problem with ordering more than one entre, side dish or desert. However, it is best to place the additional order at the same time as your main order. This helps the serving staff keep on que and at your table to continually serve you. Should you place your order for an additional item later during the meal it forces the waiter or assistant to make a special trip to the kitchen for your special order. If ordered at the time you place your original order, the additional dish will be picked up with the rest ot the table's orders and delivered at the proper time.
I often order an additional entres or other courses to try other foods I don't usually prepare myself. I even shared an order of venison with a table mate since I could not consume the whole order myself and neither could she.. There is so much terriffic food served on HAL ships it's hard to know when to quit eating. One time I ordered a leg of lamb as a second course after another hefty delicious entre. It was huge. I was already stuffed, but the lamb was the best I have ever had so I forced myself to gingerly eat it all and not waste any of it.
The dining staff work hard to offer excellent service so ordering extras with the main order just helps the staff keep on track. But, that doesn't mean you can't order a second desert that you see another table-mate enjoying during the meal. Enjoy your HAL dining experience. It 's one of the best on the Seven Seas.