Stackability varies by deal. The fine print section of any deal page will 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 vouchers, coupons, promotions, or gift cards (including vouchers from Groupon or LivingSocial).
You cannot use a voucher to purchase another deal. Vouchers are only valid toward regular priced print products.