I been playing GURPS since 1987 and the basic combat of attack, defend, damage can indeed bog down for even moderately skilled players. In reality people have options to get around the defenses of a skilled opponent and for some added complexity GURPS can emulates these options.
First off the players should not pair off with their opponents but should at least double team opponents. In the two-on-two fight this means that one player can force a step back by an enemy then the second player can strike, forcing their opponent to use their base defense.
They could position themselves so one can limit the target's defense (page 390). A side attack get -2 to defense and a rear attack permits no defense roll.
The player can choose to beat down their opponents' shields by striking at their weapons and especially the shield (pages 484, 485).
A skilled player can chose to Feint, basically a contest of weapon skill, and if the player wins the contest the difference is applied as a negative to the opponent's defenses (page 365).
A limb strike to the arm or leg can be effective as there is often little armor in those locations (page 398).
A step and evaluate can be used to gain up to +3 to hit (page 364). While it extends combat in-game time, it takes little time to resolve in table time: the player just declares he is evaluating.
Finally, historical melee combat is as much a physical contest as it is about weapon play. Slams can be used to knock the opponent down to a prone position with all his weapons unready (page 371) and at a -3 to defense. Grappling can be used to immobilize an opponent for a friend.
Look at Knockback on page 278. Basically, for every multiple of ST-2 on damage before armor is subtracted, the opponent is knocked back a hex. More critically it forces them to make a DX, Acrobatics, or Judo roll or fall down.
Don't forget to apply the shock roll once anybody takes damage. For example 4 damage means the person is -4 to all rolls until the next round (page 419). Any damage over HP/2 will force the target to roll vs Health for stunning and knockdown (page 420).
The biggest issue the player will find when using these options isn't their complexity, but figuring out when they can be effectively used. For example, trying to slam a target with a lot more HP than you wouldn't be a good tactic.
Finally, remember the NPCs' morale. Rarely will an intelligent person fight to the death. Some will run away with a just a broken sword arm – and in a two person fight, that will likely cause the other person to run away as well.
Best Answer
There's no additional rule either in Basic set or in Martial Arts. As B391 suggests, you can defend against any attack from a known enemy at -2, except for situations when the defending character doesn't know from where (and when? This is not so clear) the attack is coming.
A character with the Peripheral Vision advantage can defend at -2 even if approached from behind. What such a person sees is just a shade somewhere in the extreme periphery of sight, leading to a rough idea that someone behind is attacking. It is similar when someone jumps behind me: if I can't turn, I can just do a "wild parry" (which means -2 to the roll).
There is another interresting problem: will the fighter with an enemy in his back (assuming he doesn't want to turn, perhaps due to another enemy in front of him) be able to defend in subsequent turns? The rules are not clear on this.
I would assume the answer is no: the defender knows that someone is behind him, but he can hardly expect when the strike is coming. Anyone who ever fought with a sword (including wooden/ plastic LARP swords) would agree that timing is crucial while parrying. So my quick and easy answer is that you can only defend against the first attack. The longer answer (how I solve it in my game) is as follows, but it is rather a house rule than an official rule:
The defender can try to dodge at -4 (-2 for not seeing the enemy and extra -2 for guessing when the attack is coming; Peripheral vision buys off the first penalty and Danger sense the second if the defender wins PER vs. higher of attackers Stealth and combat skill in the turn), moving either constantly or from time to time (all-out defence or defensive attack). Parrying or blocking is trickier. The defender can choose to swing his blade wildly behind his back, allowing him to defend at -2 - but this is for whole turn, which means this is possible only when the defender chooses all-out defence (Double Defense with one hand with a weapon/ shield committed to this enemy and useless against any other attack). Or he can defend at -4 (extra -2 for not knowing the timing). If attacker chooses "wait until there's no defence", no defence should be possible unless all-out defense was chosen by the defender (the wait maneuver would waste the attack in this situation).