As long as you're buying more than one thing, you can use 2 coupons. If you're just buying one item, then of course figure which coupon is most advantageous. If the one item is less than $25, then use the $5.00 off coupon, since 20% of $25 would be $5.00.
Don't forget that BBB will take expired coupons (at least they used to - it's been a while since I've been there), so don't feel like you have to use your coupon by a certain date.