I did exactly the same as
Rachael1983 with crate training. It worked really really well for my Scottie. He's 100% outdoor potty trained now (with the exception of if he gets sick...).
I thought with my experience in crate training, it'd be a breeze when I got Casie who is 1 pound smaller. It took me a while (and a lot of hair pulling frustration) before I caved and accepted the fact that she just could not be crate trained. She would soil herself in the crate even when she had a mid-day potty break. It was a nightmare. So I gave in and let her use potty pads during the day when I was at work. I was worried that it might mess up Scottie...but surprisingly it didn't. He'd sniff her pad, but he never had accidents anywhere indoors.
Anyway...because your pups are only 6 months old, if you want to continue potty pad training them, may I suggest that you keep them in the play pen while you're away? You don't need to feel bad for them. They're ok in there. It's a lot more space than a crate and they really don't need that much more space than the 4x4. In the play pen, lay out the potty pads everywhere, covering the entire 4x4. After a couple weeks, you might start to notice an area where they
don't potty - remove that pad. Then after a couple weeks, check that they continue to not soil in that spot and take notice of whether there's another pad where they don't potty and remove that one. Then before you know it, there'll be one pad of preference for pottying on. It might sound easier than it is, but give it a shot.
Potty training requires a lot of patience and persistence...but you'll get there eventually!