okay yung design, pero it only needs some reconfiguration of both router.
you can use the windows TCP/IP gateway metric, usually naka-automatic ito, change it to manual.
here's an example of what you need to do...
assign the PLDT internal IP to 192.168.0.1
assign the belltel internal IP to 192.168.0.2
all workstations should be on the same subnet.. example 192.168.0.10 to 192.168.0.20 assigned to all workstations.
if you have 10 workstation you can configure the 5 workstation to use pldt and the other five to use the BT.
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5 workstation that will use PLDT and make belltel as an automatic back-up link.
now configure the 5 WS to use the pldt gateway 192.168.0.1 and under the TCP/IP properties go to advance setting, make sure the default gateway is 192.168.0.1 with a metric of 1, then add a second gateway of 192.168.0.2 (belltel) and make the metric as 2.
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5 workstations to use belltel and pldt as a backup link.
configure the other 5 wokstation to use the gateway of 192.168.0.2 with a metric of 1, and add another gateway of 192.168.0.1 (PLDT) and the metric as 2.
what these gives you is, the first five computer will use the pldt as the primary gateway and if the pldt goes down it will automatically switch to belltel, also this will provide some level of balancing, if the pldt gateway is very slow the computers will switch to belltel automatically.
the same way will happen to the other five that uses belltel as their primary gateway.
let me know what happens.