- Dual 12/2 is not legal in a 1" conduit. (to be more precise, 12/2NM is not allowed outdoors, and two 12/2UF are too big for a 1" conduit - that's why they bind.)
- You can't run 2 circuits to an outbuilding of the same voltage and usage. It would be OK if one of them was on a switch at the house, or had some other characteristic distinguishing it.
So the existing arrangement has been illegal for some time.
At least some of your conduit should be Schedule 80, so you also cannot run any /3 UF cable larger than #12. It won't fit, and will be a nightmare pull anyway. You probably don't know enough swear words :)
What you can do is install LARGE junction boxes at each end of the conduit run. Then run common indoor NM cable of your choice of size from panel to conduit, transition to THHN wires inside the box, run THHN wires up to #4 copper in the 1" conduit (with a #8 bare ground). Then at the other box, transition back to NM cable for the run to your future subpanel.
That should set you up for 80 amps @ 240V.
If you go #6 copper THHN + #4 cable outside the conduit, that'll let you breaker at 70A. #8 THHN + #6 cable will let you breaker at 50A. 6/6 will give you 60A. 8/8 will give you 40A.
I realize THHN individual wires is probably "new to you" and so you are not comfortable with it. Take the time to get comfortable with it!! Once you do, you'll laugh at how stupid-easy it is pulling fat wire like #4, when pulling thin cable like #12 was so darned hard!
A couple details.
Subpanel size: Go big. Go really big. Spaces are cheap, regrets are expensive. A lot of people just count on their fingers how many breakers they'll need, and look for a panel with X/2 spaces X circuits. Don't do that. Double, even triple the number of circuits you think you need today. Get 2*X spaces 4*X circuits. Disregard the number of circuits claimed; that number is a lie. If you shop carefully, and watch out for "value packs" that include breakers also, you'll find the difference for a much larger panel is like a pizza. And that will pay back dividends everytime you add a tool or device. Don't be the poor schlub who spends $200 too much buying needless wire size, but saves $30 by getting too small a panel.
Wire color markings. Use bare copper for ground wires. #10 for up to 60A, #8 for up to 100A. Neutral wires must be actual white wire, unless the wire is #4 or fatter, in which case black is fine. Mark it with white or gray tape to indicate neutral.