Using one bike for road and TT can work. There are ways to make each one more optimal, depending on how often or quickly you want to swap between them.
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The best way that is fast and super convenient is the RedShift Switch Aero System. I got one from their original Kickstarter and it is one of the best purchases I have made.
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- If you have a 27.2mm diameter seat post, it allows swapping from a road position to a TT position in seconds. Is rotates forward and locks into position. It returns to the road setup in the same time.
- The aero bars start with a main clamp that stays on the bars. Then you can quickly install or remove the bars anytime you want.
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For other bikes, you can get 2 seat posts, install a seat on each (and possibly a TT focused on on that version) and swap between them.
The overall issue is that your TT and road setup should be different with respect to your saddle height and fore/aft position. As mentioned above, using a road setup with aero bars is not optimal. I can go a bit deeper into the specifics of the differences, but won’t spend that time now or unless it is needed.