Aligning my 2 wheeler

Today I got the second wheel for my unicycle, a UDC 2 wheeler extension kit. I’ve put it all together, I think properly but I’ve found that the 2 wheels dont align properly. When I go (I say I go, I really mean push) forwards, e.g. cranks going backwards the top wheel moves the one side of the bottom wheel and when I go the other way it moves to the other side. Is this normal? Have I put the things together wrong or is something else dodgy. The “top” unicycle is a very cheap ebay one, one of these to be exact. Could that be the problems? I’ll try and post photos later if I get a chance.

Sorry if this has been covered elsewhere but I searched and couldnt find anything

You need to run the pressure in the top tire quite low to get the top tire to adopt an almost concave profile to fit round the bottom tire, but make sure it is bigh enough that the two tires don’t slip.
The top tire isn’t supporting any weight so it only requires enough pressure to make it push against the bottom tire and make it go round.
Set the pressure in the bottom tire to the normal presure you would have for your giraffe or unicycle.
If you have both tires at normal pressure they will try to go to one side like you describe.
It is best to use two similar tires on the wheels (knobbly tires don’t work too well) - maybe change the tire on the cheap ebay one for a wider tire like a 20x1.95 inch, it will give you more contact (swap the seat at the same time for something comfortable while you are at it)
When bolting the extension onto the unicycle make sure the bolts are as tight as posible without crushing the bearings (stopping the wheel spinning freely)