If you are already at greater than x3 on both products then you could give a coupon to all those that purchase a Slider for "x" amount off of the next purchase of a full Sammie. This is a method we use to track how our "promotional" items work. Number of coupons redeemed tells us how many people decided the promo item was good enough to come back for the "full sized" version. We get close to 100% redemption.
Once you do some data analysis you can decide which items are more beneficial and subsequently drop the coupon promotion.
If your cost allows, you could try $1 off the next "full Sammie" purchase. (I only suggest the $1 amount since we try to keep all of our prices at even dollar amounts to avoid making coin change....we include tax in our price as well). You might even try moving your Slider price up to an even $3 to help offset the coupon redemption cost.
Is your $5/$2.75 pricing for the sandwiches only? No chips, etc included? Just curious since our price is for a Sammie ONLY. ALL sides are priced a la carte.