This is how I do it.
I have 2 internal bool tags, conn_1_ok and conn_2_ok
I have 2 PLC integer tags, conn_1_moving_banner and conn_1_moving_banner for PLC1 and PLC2 respectively.
In the PLC programs, the tags are cycled through 0-10000.
In the HMI, on valuechange on conn_1_moving_banner I use SetValue = 1 (true) on conn_1_ok
In the HMI, on valuechange on conn_2_moving_banner I use SetValue = 1 (true) on conn_2_ok
In the HMI, on System Event 140001 (connection disconnected) I use SetValue = 0 (false) on conn_1_ok, and I use SetValue = 0 (false) on conn_2_ok.