First of all, once a system is tight, you probably don't need much of a turbo. Even though my own tank is fairly huge, I really don't need the 500 liter/sec pump I have, though "it's nice to have plenty when it's time to go from 1 bar". Probably 100 l/s or even less would do for me, and in fact, nearly all the time I have mine spun down to where that should be around what its ratings are.
There is an enormous difference between turbo and turbo drag. The latter is super easy to deal with, and I have a Pfeiffer OEM pump station that uses a forepump that looks like it came from the aquarium store - they are that good in re compression ratio and usable back pressure.
A plain turbo pump needs backing more like an oil diffusion pump, and the trick I use may not work, or might require a big reservoir and more vacuum gages (my cheapo pirani gage would work for that). If you have one of those ebay specials, it's probably this kind, and they need a real decent vacuum at the forepump side to really do much - and the lower that number, the harder it is to do the intermittent thing with the backing pump. Most of the ebay specials seem to have come from the semiconductor fab biz, and a lot of them aren't in too bad a shape or are easily cleaned up and repaired, with new bearings available - but a lot of times just cleaning it up and re lubing the bearings (with the right stuff!) is all they need.
There are two reasons to do that if you can. One is that those things wear out a lot faster than turbos. The other is they get hot and waste a lot of power (matters more to me perhaps as I'm on solar and batteries). If the oil type, one has to take a lot of extra care to prevent "suckback" when the pump is stopped. There are special setups sold commercially for that.
Here, I "had too much money" and for the big setup, just used an oil free two stage piston pump that is almost good enough to do fusor by itself. Fusors (depending on your choice of operating conditions) are run right about where a really good two stage oil pump limits out...or close. The thing is, if you want purity, you need to get a lot better than that - if half your gas is "something other than intended" you'll get kinda funny results. I'd advise you to be able to hit 1e-6 mbar *at least* and 1e-8 if you can. That latter is about as good as you're going to do if there is *any* viton seal in the system on the high vacuum side. It diffuses water, our big enemy (damn sticky polar molecule that has lousy recombination times and eats things when ionized, and takes forever to get unstuck from the chamber walls). Butyl doesn't, but diffuses air instead...CF doesn't leak AT ALL, but it's a trick to reuse the gaskets - you don't tighten them fully the first time, leaving some room to tighten a little more next time, and you can get 3-4 times out of one. Not that big a deal with the cheap ones for 2.75" but huge with the expensive big ones. The reason the flanges are so thick, with so many bolts, and they tell you to just pull it in to touching is that the directions are for academics and grad students who will screw up anything requiring real-world judgement. (I'm sure that's not going to make a lot of friends, but I've been there and observed that)
Don't test run turbos with 1 bar on either side, and especially be careful about inrush accidents if one is up to speed - not only can they fly apart with the energy of a car crash, the blade tips are hypersonic and can melt...don't ask how I know this. It's the most horrible sound you've ever heard, combined with money going up in smoke.
For intermittent backing, what I use is a sensor (it's built in to pfeiffer turbo controllers) for how much power the turbo is using. Above some threshold, run the backing pump, below that, shut it off. Works a charm. If you don't have a little hysterisis, the "hunting" the controller does to regulate speed will give you some fits, but a retriggerable one shot driving your solid state relay should take care of that one. Most times it will take only a couple revolutions of the backing pump to be good again, though of course it'll run for half an hour when pumping down from STP.