Use another MacBook as extended display via Thunderbolt 2 cable?
ScreenRecycler enables you to use your MBP 2013 as an additional display for your MBP 2015 laptop. It's not very expensive (about $30).

Both computers need to be connected together through the local network (can be either Ethernet or WiFi, or maybe even a Thunderbolt Cable if you really want that).

The MBP 2013 needs to be running Mac OS X 10.8, 10.9, or 10.10 (because ScreenRecycler is not compatible with 10.11 and newer).