G'day Jason, 

You won't be able to carry two X mugs inside the X- Set 21, as the lid, won't quite close. The X- Set 21 will nest perfectly inside the 8" Alpha Pan. The X Plate is too large to nest inside the 8" Alpha Pan.

G'day Heather,

Yes, there's no reason they couldn't be used on your indoor stove, but it's a good idea to be mindful of your flame.  Keep it low enough so it's only in contact with your anodised aluminium base and away from lapping up the sides of your pot.  The silicone sides are heat resistant to the food you're heating up but we wouldn't recommend exposing them to direct flame.  Cheers :)

G'day Jennifer, 

I've just checked with the display model, and as the pot that comes in this set is the 1.4L size X pot, you won't be able to fit a second X bowl in there as the lid won't close over it.

The X Set 31 has a bigger 2.8L pot that will fit two X bowls (plus to X cups) so that might be one to consider instead.