I often have joint bday parties for my children and gifts are rarely an issue. It's usually really close friends & family being invited so they'll already know anyway. For the kids friends being invited, I send separate invites for only that particular bday child because I certainly don't expect them to bring multiple gifts for the children. My kids are pretty close in age, and I don't really plan a lot of games, etc. In the summer, we just have them at the park so the kids can run & play to their hearts content although, we usually have a pinata. As for decorations, it varies year to year. I usually just have a happy birthday sign up somewhere, each child's birthday banner, and the special helium balloon they picked out for themselves (that sits on the table for their gifts). If there's a particular theme they wanted, besides picking it up in the cake for each child, I buy wrapping paper in that theme to cover the tables with because it's so much cheaper than the themed tablecloths and it's sooo easy to clean up. Like I said, the cakes are usually separate but last year we were doing 4 people (2 of my children, my husband, and my nephew) and I made a huge sheet cake with a lincoln log castle at one end and figurines such as dragons, knights, & princesses to represent each bday person. It was so much fun and people only brought gifts for the person they were invited for. There was one neighbor I had invited that had missed it was a joint party so she felt bad she only brought something for one child but my daughter hadn't even noticed that she didn't get anything from her. The neighbor didn't feel bad about not getting anything for my nephew though since she'd never met him nor did she get anything for my husband.
Since this is a first bday party, you don't really need to do much. For people who aren't family or joint friends, just send separate invites since they can't be expected to bring a second gift and there are people out there that would because they weren't sure if they should or not and they wouldn't want to look stingy. It's not nice to be in that position. I was and I wound up bringing a gift, which is fine since I'm really good friends with that mom now and she loved the craft apron I made her son. I wouldn't change it since that's how we really got to talking but I certainly don't miss that feeling of uncertainty about it. When it comes to gifts being mentioned on the invite, I've always been taught it's OK to say something along the lines that gifts are not expected so it gently lets people off the hook, especially in this situation if you choose to send out joint invites. Congratulations on your little ones birthday!