if company A buys office supplies for both companies on the same bill
say 100$ for company A, and 200$ for company B

company A GJ
db 100.00  5700 os        (not required if supplies only bought for company B)
db   5.00  2315 gst paid  (not required if supplies only bought for company B)
db 210.00  3104 interco B (would be the full amount, if supplies only bought for company B)
cr 315.00  1065 bank

company B GJ
db 200.00  5700 os    
db  10.00  2315 gst paid
cr 210.00  2234 interco A

now company B owes company A, 210.00
====================================
company B GJ
db 210.00  2234 interco A
cr 210.00  1010 bank

and company A records the payment
=================================
company A GJ
db 210.00  1065 bank
cr 210.00  3104 interco B

============================
as to your second point
  I am not sure where the initial payment was made from.  It may have been included in another transaction.
  I cannot see this specific amount coming out of the Bank in company A.    
  I do see where the payment from company B comes into company A's bank.

what was the method of payment when the supplies were purchased ?
bank debit card ? cash ? cheque ? credit card ?

if the payment is c/c, the first GJ would be
company A GJ
db 100.00  5700 os        (not required if supplies only bought for company B)
db   5.00  2315 gst paid  (not required if supplies only bought for company B)
db 210.00  3104 interco B (would be the full amount, if supplies only bought for company B)
cr 315.00  2xxx credit card payable

if company A buys office supplies for both companies on the same bill
say 100$ for company A, and 200$ for company B

company A GJ
db 100.00  5700 os        (not required if supplies only bought for company B)
db   5.00  2315 gst paid  (not required if supplies only bought for company B)
db 210.00  3104 interco B (would be the full amount, if supplies only bought for company B)
cr 315.00  1065 bank

company B GJ
db 200.00  5700 os    
db  10.00  2315 gst paid
cr 210.00  2234 interco A

now company B owes company A, 210.00
====================================
company B GJ
db 210.00  2234 interco A
cr 210.00  1010 bank

and company A records the payment
=================================
company A GJ
db 210.00  1065 bank
cr 210.00  3104 interco B

============================
as to your second point
  I am not sure where the initial payment was made from.  It may have been included in another transaction.
  I cannot see this specific amount coming out of the Bank in company A.    
  I do see where the payment from company B comes into company A's bank.

what was the method of payment when the supplies were purchased ?
bank debit card ? cash ? cheque ? credit card ?

if the payment is c/c, the first GJ would be
company A GJ
db 100.00  5700 os        (not required if supplies only bought for company B)
db   5.00  2315 gst paid  (not required if supplies only bought for company B)
db 210.00  3104 interco B (would be the full amount, if supplies only bought for company B)
cr 315.00  2xxx credit card payable

Roger L said:

now company B owes company B, 210.00

Sorry to be picky but Roger, did you mean

now company B owes company A, 210.00?