Trogir to split

You can get them that day. There is also a ferry from Trogir to Split you can check out at Croatiaferries.com. Look under ferries from the Croatian mainland. Takes about the same amount of time.

Thank you, I will look at the ferry, but if we decide on a bus.. can we buy tickets in advance ? Securing seats ... I ask because I googled, some sites allow to book in advance.. is that a good option ????
Also, we would love to visit mostar, ( no car ) day tours look OK,(ish) we would rather spend more time in mostar if the option is available... I asked on trip advisor , only one reply saying too far for a day trip because we spend 10 hrs on a coach ! ?
Thank you again

Mostar is definitely worth an overnight stay, especially if you're going by bus from Split and back. If you have time to add a night there, definitely do, and I highly recommend staying at Muslibegovich House (https://www.muslibegovichouse.com/). However, their website seems very out-of-date, so probably best to book via Booking.com (https://www.booking.com/hotel/ba/bosnian-national-monument-muslibegovic-house.html).

Busses run regularly between Trogir and Split, and most are just normal local transit buses. You can buy tickets at the station and just go. For most of those buses buying a ticket in advance does not "secure a seat".

The one thing to be aware of is that Bus 37 (via the airport) ends at a terminal a bit distance from the old town, which is probably where you want to go. The less frequent long distance coaches go to the coach terminal at the waterfront, which is closer to the old town.
Just you should be able to just get a ticket and get on. That is what I always did when in Croatia.