Looks like a home-made tandem to me.
Here are my reasons:
- The captain's seat post has no clamp. Instead there's a hole, presumably for a bolt to go through the seat post. That's not done on a purpose-built tandem. Here someone's cut the clamp off to allow the top tube to continue.
- I see a tang at the bottom under the Keel Tube for a chain tensioner. A purpose-built tandem would be more likely to use an eccentric BB in the front for tension
- That tensioner seems to be on the right-hand side of the frame - a normal tandem would have the timing chain on the left side, and would have special bottom brackets to account for this.
- The stoker's position has a head tube of its own - normally a tandem would have a special stem that secures to the captain's seat post, bringing both riders closer together. The larger the gap between riders, the less benefit you get from aerodynamics.
- The welding on the stoker's BB housing looks different for the keel tube - specifically its a lugged frame but the keel is butted straight onto the lug, not into a recess.
I suggest you look closely at the rear of the captain's BB shell, for the remains of chainstays that have been cut off. I do see the stoker's head tube has been extended with new metal that doesn't match, that will also be welded in. You could also look at the sides of the stoker's downtube for the remains of shifter bosses.
- The tube diameter of the additional pipe in the middle is higher than the rest of the frames - so between captain's seatpost and stoker's head tube is noticeably wider tube, as is the keel and the lower stoker's head tube.
The paint job looks like a can of spray paint, specifically where the white fades into the red. To be fair that's a really hard transition to paint well.
So either the frame builder added some stickers after painting, or they welded it such that the original stickers stayed usable, and only the new welded areas got painted.
Please don't take this as talking down your frame - it will be a loverly bike when you've finished building it. But don't waste too much time trying to identify the brand, when it could have been made by someone in a home workshop.