I was just wondering how old your babies were when they formed regular times for daytime naps? I think ds1 was about 7 months when he did this. Ds2 is 5 months and I can't much sign of this happening...
My 5.5 month old LO fell into it gradually between 4-5 months. It's not an exact routine, as it can vary by an hour or so each way from day to day, and it depends on what time she wakes up in the morning. But she seems to need to nap roughly 2 hours from when she last woke up, so she's napping at approx 9am, 12 and 4 (ish). I didn't try to push her into it, I just gradually realised that this what what she needed as she was getting very very grumpy if she was awake for more than 2 hours at a time. But they are all so diffierent - my friend's LO who is almost 6 months is showing no sign of following any set pattern with naps and seems pretty happy with this.
DS2 is just getting a vague routine now and he's four months. The only real certainty is that he goes back to bed at around 8am for two hours, the rest of the day is hit and miss but usually involves another two hour kip at about 12.30pm and then a short one about 4pm.
Bit it all really depends whether we're out and about or not.