Plumbing – Will a doubled P-trap cause problems if it has a vent between the traps

Much too much up and down! You want a P trap with an additional angled connector toward the wall.

The first down and up from the sink drain is correct. That is the P trap. It should end with a horizontal leg pointing toward the wall (an L shaped piece).

This will not line up with the opening in the wall because the drain is offset. Therefor that leg will run a distance and then will use an angle to bend it back toward the wall.

You could use a right angled slip joint connector (running horizontally).

A 45 degree angle would be better (the less sharp the bend, the less likely to stop up). However it may be hard to fine a 45 degree slip joint elbow. You could use standard pipes and elbows, but these would require glued joints rather than slip joints.

Finally there should be a straight run from the angled connector to the wall connection.

You can rotate the P part of the trap so the L leg coming off the P is parallel to the wall (assuming you are using the 90 degree elbow; if not, adjust accordingly). Then the elbow turns the drain toward the wall. Another horizontal run carries the drain back to the wall.

You need to measure the length of the horizontal leg of the P and the length of the connecting run to the wall; be sure to allow for the sections embedded in the elbow and trim each accordingly. Assemble.

It is easier done than described.

You should have a trap generally the trap would be with in 8 to 12 inches of the drain. The trap prevents critters (at least partly) and sewer gases from entering the home from shower drain.it sounds like you have an AIR Bubble and until that gets filled - it acts like a blockage. Next time get a bucket of water and force it into the drain and then see how fast shower drains.

It is possible it could be a venting issue you are not up the creek. You mention the Toilet being close by and I would guess the two drains are connected and the vent for the toilet exists, so you might need to enlarge it or see if by chance it has become clogged.

Also given that you dug all of that out check that pipe for clogs and also your venting pipe where it meets the toilet drain it is very possible you have a piece of debris stuck and it rolls around a bit and out of the way after it 'floats' (moves).