Frequency of 856 from Yishun and WRI is at 05-06 mins now after
the new round of BSEP add. So possible that if one is late to
depart and gets caught up, you might see 2-3 buses bunching quite a
regular scene.
I am okay with 2 HC buses bunching on 856 bcoz the loading b/w
Marsiling MRT and Admiralty Prison is v high, similarly loading is
very high in Seneko area in both directions. The loading b/w Yishun
and Sembawang is quite low though. 
Erm back to back, not bunching lol..
But they arrive quite on the dot now..It's like there's no Bendy
or A22s in between also..
Infact, this svc should have more Bendy/DD, <5 A22s...