to much gain

having two pedals shouldn't take a toll on your amp, but the gain levels may get extreme to the point of ruining your speaker. If you want those rumbly riffs, boost the bass and play in a dropped or flattened tuning. (never tried it, actually, but theoretically it makes some sense)Sometimes you can get way too much gain, which isnt really pleasing to the ear.

If you have insane gain on your pedal(s), then you should only need one.
I sometimes use a tube screamer with the distortion lead channel on my buddy's 30 watt marshall, but i use very little gain on the pedal.

Great suggestions from Psychonik, scotty. If I understand your post correctly, you have 2 pedals and you use each one for a different thing - sorta like having 2 separate 'patches' or presets almost. I had a similar thing years ago in cover bands I used to be in. With a channel-switching amp you get basically two distinct sounds, clean and lead. Add one distortion pedal and you get 4 sounds, clean channel with distortion on top and lead channel with distortion on top. Add another pedal and you get another bunch of 'presets.' It's a cool way to go actually.

But to keep things managable, I would sit down and play with each pedal and set each one so that the volume change between 'out' and 'in' were almost the same, just listening to it. My main one was an MXR Distortion+, which is more or less just a fuzz pedal. I had it set so when it was 'in' or activated, it was just a tiny bit louder than when it was off. Just slightly noticeably louder. Then when I punched it in, using the clean amp channel, it would be pure fuzz (to emulate synths), and when I clicked it in using the lead/distortion channel of the amp, it would focus the sound and put an edge on it, though not making it that much louder. The lead channel was always set just a tiny bit louder anyhow.

The second pedal I had was an Ibanez SuperMetal pedal, which was set totally different than the Distortion+, but with the in and out volumes similar to the Dist+. I used it by itself with the clean amp channel or in conjunction with the other pedal. You wind up with a whole bunch of different sounds combinations. But at times, like the last song of the night, or for some insane noise type solo, I'd click both pedals on, switch to the lead channel and let everything go crazy. Still though, because I made sure each pedal's 'in' volume wasn't too much, the increase in overall volume with them all on wasn't enough to do any damage to the preamp circuit of the amp or blow any speakers. It just 'sounded' really incredibly loud. The master volume on the amp determined the actual volume.

Anyhow, the idea of using different pedals to get different sets of sounds is a good one. Just keep the in and out volumes pretty nearly matched and you shouldn't have to worry about your amp.

Take care

Running two distortion pedals at once should not hurt your amp, I have done it many times. Maxing your amp out is the best way to blow it.

The problem is that super high gain sounds great for rhythm guitar but not lead. The tone is too thin and airy for lead. What will work is an overdrive pedal like the Ibanez Tubescreamer. I own a Danelectro Daddy-O and am very pleased with it.

If I need modern high gain distortion for a modern grunge type sound I use a Danelectro Fab Tone for that. Then I have the Daddy-O set to boost the volume up for a solo. But I turn the gain down quite a bit on the Daddy-O. So I am using two pedals at once. The result is I get a very clear, warm, thick tone to solo with. It really cuts through the mix of a band and can be heard well even with other guitar players playing. I usually place the Daddy-O after the Fab Tone in the chain. I also use my Fender HRD Drive and More Drive channels in the same way. This works well for me.

Check into the Ibanez Tubescreamer. They make a re-issue of the original TS-9. This is the most famous overdrive pedal ever. The re-issue is supposed to be just as good as the original.

Ibanez TS9 Original Tubescreamer Re-Issue