I feel more comfortable with mine in the shower alone than the bath. Dd is 4 next week & I feel ok with her in the bath alone at waist level. DS likes it deeper, so even though there are 2 of them, I'm just as wary as if it was only dd. I leave the door open & listen to everything. Call out if it's too quiet.
I think it's been the last 12 months or so I've been leaving her alone for longer.