Author Ivan Sorenson describes many different options that serve to promote social interaction and roleplaying instead of just combat, and this is one I've chosen to showcase for adaptation into BECMI. Consider these optional rules applicable to all PCs.
Blade Brother (or Sword Sister, for the female PCs)
PCs who fight together quickly learn each other's tactics, strengths and weaknesses, sometimes creating an almost supernatural bond that aids both of them in combat.
Any two PCs, regardless of class (the DM may require them to be of the same alignment, I do) may choose to become Blade Brothers. After this declaration, one game session must pass before any benefit is earned, as they study each other's combat techniques. Upon the next game session, both PCs receive the following benefits:
1.The partner who goes second in combat in each round receives a +1 bonus to hit if the first partner's attack was successful. No penalty is invoked if the first PCs attack misses, however.
2. At any time during a combat, one partner may supernaturally "loan" HP to his partner, though this is a full round action for both PCs. To transfer the HP, the partners must grasp hands and vigorously proclaim their battle cry, at this time, the PC giving the XP suffers 1d4 HP dmg, which is added to the recipient PC's current total, not to exceed his maximum HP.
3. If one of the partners is slain in combat, his partner suffers a -2 penalty to hit and damage for the rest of that session of play (or 1d3 days, if time is passing quickly that session) to reflect the supernatural loss and remorse. Avenging one's partner's death by defeating his killer in solo combat removes this penalty immediately.
A PC may only be bonded to one other PC at a time in this fashion, though he may choose another PC with whom to bond if his partner dies or retires, or the bond is dissolved. A retired partner inflicts no penalty on the remaining PC, although a bond with a new partner may not be forged for 1d4 weeks of game time, and the 1 session bonding period must then be repeated for the new partners. A bond that is dissolved mutually by the two PCs invokes the same penalties as the death of a partner, in #3 above, on both partners, but without the vengeance loophole.