I have two bathtubs, that are both slow to drain. We've used several types of drano, etc and they've all worked a little. But this morning my husband took a shower and the water didn't drain at all. Then we realized that the water had backed up into the bathtub as well. I'm worried that the main line might be clogged... but I don't know where to go from here. Any suggestions for something that I can try before calling the plumber?

Sounds like you have back to back bathrooms with both tubs draining into the same branch. This makes it easy to clear both tubs at the same time. Unscrew and remove the overflow plate,(The one with the drain lever) and pull out the tub stopper. You now have a clear shot at the trap. The way you were going in through the drain it ran you square into a tee that you can't get around. Now feed the cable( 1/4" best or 3/3" with a small tip. You can rent a Ridgid K50 sink cable.) About 2' down you will run into the bend of the trap. Crank and push at the same time to work around it. Your blockage will be found in the lateral pipe going to the stack. You shouln't have to put out more then 10 or 12 feet. I have found tub and shower clogs to be mostly hair and the worm should auger into it and pull it back. Hope this helps and thank you for rating my reply. TOM