Discussion: Can You Use a B1G1 Coupon + Another Coupon?

I get a lot of questions about this topic, and so I thought I would bring it up on here and see what some of your experiences have been with this topic:

Q: Can you use a "Buy One, Get One FREE" coupon with another manufacturers coupon for the same item?

Apparently, this is a YMMV (Your Mileage May Vary) situation. Some people claim that the extra $-off coupon is taking it off of the one you are "buying" on the B1G1 coupon, and the B1G1 coupon is for the FREE item. For example, right now, many blogs are sharing these two deals:

Scrubbing Bubbles Toilet Gel
4-in-1 Scrubbing Bubbles Starter Kit
$2.50 (Toilet Gel price at Walgreens)
- Buy a Toilet Gel, Get a 4-in-1 Starter Kit FREE (Printable Coupon Here)
- $1.00 Off Toilet Gel (Printable Coupon Here)
= $ .75 each after both coupons!

Nivea Lip Care Product
$1.50
- Buy One, Get One FREE (June All You magazine)
- $1.00 (Printable Coupon Here)
= $ .25 each after both coupons!

I have been particularly confused by this issue lately, as it seems that sometimes I can use two coupons like this with no problems, and then other times, the register will beep.

So, I am interested to hear about your experiences...have you all had success in using both a "Buy One, Get One FREE" coupon and another $-off coupon on two items? Please let us know in the comment section. I think a good discussion on this might help some people out.