What you can do is get a glove of storing, which allows you to store and retrieve an item as a free action. On your turn, deactive your ring of force shield and retrieve your weapon from your glove of storing, attack, and at the end of your turn, store your weapon in your glove of storing and activate your ring of force shield.
Alternatively, get Improved Buckler Defence from Complete Warrior, so that you can get the buckler's shield bonus to AC and still attack with a weapon in your off hand. This imposes a -1 penalty on all your attacks, but a buckler can potentially provide a better shield bonus to AC (up to +6) than a ring of force shield.
By the way, a rapier is technically a one-handed weapon, not a light weapon, so even with Two-Weapon Fighting, you will attack at -4 for each weapon. To enjoy the lower attack penalty, use a short sword (lower crit range), a kukri (lower base damage) or an elven lightblade (also from Complete Warrior, requires Exotic Weapon Proficiency, or if you are an elf, you can get proficiency with all elven exotic weapons with Improved Weapon Familiarity).