Can I combine (stack) vouchers together?

Stackability varies by deal. If you look in the fine print section of any deal page, it'll let you know if the voucher is limited one per order or stackable. 

Collage.com percentage-off vouchers/sales, such as the 60% off first-time buyer discount, cannot be combined with other deal vouchers (including vouchers from Groupon or LivingSocial).

You cannot use a voucher to purchase another deal. They are only valid toward regular priced print products.