That sounds like a perfectly acceptible way to train to me. The only thing is now you have to be clear when you're teaching the dog and whenever you use the commands in the future, that a sit/down does not only mean to do those behaviors, but also to stay put until released. It might take a bit longer at first because you're kinda asking for two different behaviors at once, but it's a good concept and can probably be beneficial in the long run if you compete in events. Sometimes less is better.
Personally, I use the stay command. I like to ask my dogs to sit/down before treats/dinner etc. and don't expect to have to release them afterwards when they've received their reward. I also use 2 commands for a stay. I differentiate between a STAY, where the dog needs to stay put until I return and release it, and it will usually be an extended time frame. And then I use WAIT for when I might be asking for a different command from the stay and not returning, or just as a more informal stay where it doesn't to be so precise and long. If I'm doing something with them off leash somewhere and they get a bit ahead of me I like to use the wait so they pause a second and let me catch up to them a little bit. lol I've heard some people have success with this, so the dogs differentiate between the long sits/downs in obedience and the sit/stay before a recall. Using a different word for the longer stay will let the dog know it can relax and wait as it won't be asked to do something quickly soon so they hopefully won't anticipate a command and break the stay.
I also use two different come commands. COME is for the precise obedience recall where the dog needs to come to my front and sit squarely at my feet, without the additional sit command. And then I use HERE as a more informal command for the dog to move closer to me but the position is not specific and there's no need to sit. The here works well for agility work where the dog needs to take an obstacle towards me, or if they're getting to far away. Or for general use inside and out when I need the dog to move in or pay attention to me and do something.