Simms scored a brace as the Black Cats secured their first win of the Championship season, with Stewart scoring the winner in the second half.
Head coach Alex Neil hailed the pair as 'unbelievable', adding that they had 'run the defence ragged'.
Simms says the pair will only get better as they play together more.
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"We're both strikers who like scoring goals and like helping the team out as well," Neil said."Hopefully we can build a great relationship together whether it is playing together as a two or whether it's one of us starting, we can keep pushing each other to get the best out of us."We both played and both caused them problems, so I just think it's only going to get better."I'm quite versatile, and likewise with Ross. I think it gives the manager the option, we can play together but we're both capable playing up there on our own."We're already building a good connection."We're both quite big lads and we both like the running, so it's just about trying to work off each other. The manager just said to do what we normally do, I think we did well."
Simms almost had a hat-trick, denied only by a brilliant save from Daniel Bentley.
"To be fair to the keeper, that was an unbelievable save," Simms said.
"I thought I'd done everything right, but the main thing is that as a team we got the three points.
"It was a dream debut.
"When you go down away from home it can be difficult, but we didn't get flustered.
"There's a real bond in the team, a togetherness. So we were never going under, we just kept pushing.
"It's a great bunch of lads here, so I already feel settled. We just want to kick on."
Simms' performance led to a huge ovation from the travelling support when he was brought off late in the game, and they had been singing the name of their new striker long before that as he made a major impression.
"It was nice to hear them singing my name, I've only been here a week," he said.
"It gives you a massive lift. Sometimes it can take you a bit of time to get up and running when you're on loan, but I thought I did well today and hopefully I can build on that.
"Hopefully I can keep them signing."
Alex Neil says he is thrilled to have more options up front: "We always knew Ellis would be available on loan, so we spoke about how in the last month of last season we needed another centre-forward.
"One to lighten the load on Ross, two to challenge and push Ross, and three to play up next to him and give us another dynamic to our game.
"I thought you saw that dynamic today. I’m not going to compare them to Kevin Phillips and Niall Quinn because they’re miles off that, I’m not making a stupid comparison, but the point I am making is they did really well in terms of that pairing.”