If the mid silencer chamber is left in place a simple tube would probably be adequate for some noise suppression but probably rubbish for performance & the MOT tester may not like a plain pipe hence a sport bike silencer that is light and gives a deep note to the exhaust. Removing the mid chamber is more effort.
You need 2>1 Y piece, I used a mini cooper (old type) Y piece on my VT500, may fit the TAs but then you may need to baffle the Y outlet before the straight through sport silencer if you want to do a DIY job.
Sport silencers can be had cheaply at bike jumbles paid £15 for my Benelli one 3-4 years ago and £20 for the pair of R1 silencers this year.