Recently bought olimexino-stm32, mod-gps and mod-rs485.
Dunno that mod_rs485 - bridge over rs232 (my fault).
Is there possible solution to operate both mod's on same uext splitter?
There is no way to use two UART boards on the same UART line.
The only interface at the UEXT connector that is capable of handling more than one board is I2C (and each I2C board needs to have unique I2C address). You can use only one SPI and only one UART device on, respectively, the SPI pins and the UART pins.