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The advice you heard is true. They should need separate exhaust hoses. Another issue is the situation when both fans are on. Your existing exhaust pipe may not be sufficient to handle both, so they both become ineffective.
I just had this same experience with my home. I originally combined the two into one exhaust. The new fan came with a one-way valve, so air would not reverse direction, but the old one did not have such a device, so the new fan simply blew into the old one. I later was told that my/your plan is not very healthy.
You could consider just letting the new fan exhaust into your attic. A properly vented attic is supposed to be fine for this, and many newer homes seem to employ this procedure, but it‚Äôs not my preference. I ended up running a new exhaust tube to the outside, and it really was not that much work for me.