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Debian 8 and Debian 9 Dual Boot

My first Linux install was about one year ago. After some missteps, I have used Debian 8 in reasonable satisfaction on the desktop during that year. Now I want to leave 8 in place and do a network install for Debian 9 on the same disk and switch back and forth at boot time. LVM reports as follows:

dan@debian:/$ sudo vgdisplay -C
  VG           #PV #LV #SN Attr    VSize      VFree
  debian-vg   1     5     0    wz--n- 976.56g 938.20g

Currently, Debian 8 needs less than 40g for me as you can see:

dan@debian:/$ sudo lvdisplay -C
  LV             VG                Attr           LSize   ...
  home       debian-vg     -wi-ao---- 10.00g
  root         debian-vg     -wi-ao---- 8.38g
  swap_1    debian-vg     -wi-ao---- 7.81g
  tmp          debian-vg     -wi-ao---- 380.00m
  var           debian-vg     -wi-ao---- 11.79g

So there is plenty of disk space for the two Debians and more besides. The question is how to prepare to install 9? My guess is to define another volume group called debian9-vg perhaps but how will this be recognized during network install? I've clobbered stuff before during installs and I'm gun shy. Maybe there is a better way. Any thoughts on this will be appreciated.