Tomato watering in "grow pot" growbag question

I bought a couple of those growbag greenhouses this week (and some growbags..) to try some tomatoes down the allotment this year.

I've planted two tomato plants in each growbag, and have used one of those grow pot things (a sort of extension tube with an outer water reservoir) on each plant. The instructions say to water thoroughly then not fill up the outer reservoir for 2 weeks to encourage the roots to grow downwards. However 2 weeks without water sounds bad, and surely just watering the centre pot won't encourage much downward growth?

I'm tempted to use instinct on this one and water when I think necessary, but anyone else tried a similar technique? The pot in question is this:
http://www.amazon.co.uk/gp/product/B001ONHTME/ref=s9_simh_gw_p86_d2_i1?pf_rd_m=A3P5ROKL5A1OLE&pf_rd_s=center-2&pf_rd_r=1ZTQMWQ131PAYGCJSHFF&pf_rd_t=101&pf_rd_p=467128533&pf_rd_i=468294

Ta!

Yeah I'm tempted to give them another water today (the centre and the outer bit). One is looking a little limp, so probably needs more water.

I watered mine every other day through the outer ring only, immediately after planting in the growbag. Fed through the inner ring every 4 days once they were fruiting. Had my best ever crops of tomatoes, peppers and aubergines