1)
I would do it preprovisioned - just add the right serial numbers:
set virtual-chassis preprovisioned
set virtual-chassis no-split-detection
set virtual-chassis member 0 role routing-engine
set virtual-chassis member 0 serial-number XXX
set virtual-chassis member 1 role routing-engine
set virtual-chassis member 1 serial-number YYY
2)
When the two switches are joined in a VC, then you configure a LAG just like a standalone switch, only difference is interface numbering; member0 interfaces will start with eg. xe-0 and member1 interfaces start with xe-1.
Example config for a LAG on the first two physical ports in the QFX3500s with LACP enabled doing an access port in vlan XXX:
set chassis aggregated-devices ethernet device-count 10
set interfaces xe-0/0/0 ether-options ae0
set interfaces xe-1/0/0 ether-options ae0
set interfaces ae0 aggregated-ether-options lacp active
set interfaces ae0.0 family ethernet-switching vlan members XXX