Hello all! Need some advice please.

I brought two juvenile garters about a 2 weeks ago and one is very timid (Jake)so much so that it spends all his time buried in the aspen substrate.It has refused food (tried pinkies,fresh fish,worms) although i'm not too worried as i expect he's still settling in.

The other snake(Snowflake (not my choice)) spends much of it's time basking under the lamp and has just started accepting worms but only in the viv where i have found the aspen sticks to the worms and therefore some got swallowed.

What substrate should i use that can accomodate both snakes until they will both feed outside the viv?One that they can burrow in (for Jakes sake)and either inedible or safe if swallowed?