Hi Kevin,
March is a great time to visit both TN and FH, and you have enough time to do both some justice. A great warm up for Borneo too.
It is possible to bird both areas without a guide (and pre-book taxi transfers between locations), particularly if you aren't short on time - there is lots of info on various trip reports about the trails and key sites for birds at each place, and at that time of year there is a good chance you'll bump into other birders who can share gen. I thoroughly enjoyed birding TN on my own, though I am sure I would have seen more with a guide. If you have birded Asia before, and are reasonably familiar with many of the birds you will see, then I would be less inclined to hire a guide.
If you have not birded Asia before - and particularly if you haven't birded in the rainforests there, then it might be better to hire a guide. This will help filter out the calls of the birds you want to see from say squirrels, geckos, frogs etc, and help ensure you get into the right habitat for species you are more interested in. I don't know how many guides you have asked to quote, but you could try a few more. I have used Mr Lee from Endemic Guides (
http://www.endemicguides.com/) and can recommend him, and Weng Chun is also well recommended (
http://www.nature2pixel.my/). I don't think you should rely on being able to find a guide at TN.
If it was me I would split my time, spending a few days with a guide as follows:
I would hire a guide to take me from KL via either Krau Forest or Bukit Tinggi to FH on day 1, and keep the guide for my first full day on FH. Krau Forest is good for both Malaysian Rail-babbler and Garnet Pitta (though these are also both in TN), and Bukit Tinggi is a site for Mountain Peacock-Pheasant (see
https://penangbirder.blogspot.co.uk/2016/10/natural-born-models.html).
Its best to visit FH ahead of TN as it is cooler, and a better place to start acclimatising. If you only have the guide for your first full day on FH, make full use of their vehicle to visit sites that you won't be able to walk to during the rest of your stay (eg the Gap). Thereafter you could manage without a guide if costs are an issue. Take a pre-booked taxi transfer to TN (and from there back to KL), as it isn't worth hiring a car as you won't use it during your stay at TN. I would transfer to TN after two full days and a morning at FH - the birds at TN take a bit more work to find.
If budget isn't such an issue, then I would keep the guide up to and including my first full day in TN, particularly if you haven't birded this part of Asia before. After that I would be happy birding the rest of the trip without a guide - you will have a good feel for the trails at TN. One thing you really should do at TN is hire a small boat to take you up Sungai Tahan as late in the day as possible, making sure they know you want to drift back down river without the engine on. Ideally you want to be coming back down the river well after the last tourist boats, this will give you a much better chance of seeing the riverside birds along here. If you stay at the resort you are on the right side of the river for early and late starts, but I would recommend eating at the floating restaurants rather than the resort restaurant - cheaper and more interesting food.
Enjoy Malaysia, Paul