Replacing exhaust flexis S4

[TED7]

One of my flexi joint braids has come undone completely at one end and is being held together by the inner pipe, as a temporary fix for the time being due to funds (thermostat also needs replacing, wheels need a refurb, car needs lowering, milltek, remap etc. Usual S4 problems), I intend on chopping out the worn flexi and replacing it with a slip on part and sealing with putty. Is using putty before the catalyst a bad idea given it's a bit brittle when hardened?

Also given that we are supposed to replace the turbos in pairs, and my flexi botch will probably affect gas flow on one side, should I replace both flexi pipes so I have a balanced bodge job?

Thanks.