While it is possible to tie the VBIAS and EXTVcc pins together on the LTC3897, there are important considerations to keep in mind. EXTVcc should not be connected to a voltage higher than VBIAS. VBIAS can handle up to 76V, whereas EXTVcc is rated for 14V.
Connecting them together defeats the purpose of EXTVcc as a backup, making switchover between the two impossible. The primary advantage of EXTVcc is that it allows DRVcc power to be supplied from a high-efficiency external source, thereby reducing the power dissipation of the VBIAS LDO.