I soldered BNC connector to the Digi+ Pro board
https://www.hifiberry.com/shop/accessories/bnc-jack-gold-plated/
Digi+ Pro board has now two S/PDIF electrical outputs. On both signal is available.
Can both electrical outputs be used simultaneously? Does use of both electrical outputs influence/deterior signal quality? Coax cable with 50ohm are used.
No, you can only use one, otherwise you will mess-up impedance completely. You should also use 75Ohm cable as SPDIF is design for 75 Ohm cables.
Sorry, I meant 75Ohm. I use this coax cable
https://www.koax24.de/storage/datasheet/de/050245_Datenblatt_RG59BU.pdf
And what about using BNC and TOSlink same time?
Is it possible to modify Digi+ Pro with BNC that TOSlink and cinch coax are switched/cut off when BNC is in use? May this improve signal?
Yes, TOSLink + one electrical output work well.