Dear Amrita,
Yes, you can add the two builders as follows:
left_lead1+=left_lead2
but in the new version of kwant, the operator += for builders is deprecated. So, you better use:
left_lead1.update(left_lead2)
and then you add your missing hopping, before you attach the lead:
left_lead1[(lat(0,0),lat(0,-1))]=t
I don't know why you separate your lead into two parts and then build it again, but you can get it directly in one shot.