It must be noted that there are several dual VoLTE phones that support 4G VoLTE on both SIM slots but only on one at a time. Dual SIM dual VoLTE (DSDV) phones can keep two 4G or 4G VoLTE SIM cards active in your phone (in Standby configuration, not Active) simultaneously.
---- Not Sure abut the Volt Thing ...................
Following budget phones supports 4g + 4g and are priced below 8K.
Samsung Galaxy On7 Pro, ..... 6990/-
Samsung Galaxy On5 Pro ...... 6290/-
Yu Yunicorn ....... 6371/-
Yu Yureka Note ...... 6490/-
TenorE (10.e) from Amazon also supports Dual 4G
Following Phones Support Dual 4g and in Range of 10 K - 15K
Moto X4 .....14,700/-
RealMe 1 128GB ....14,000/-
RealMe 1 64GB ..... 11,000/-
Samsung Galaxy On Max ....12,000/-
Oppo F5 ...... 13,300/-
Nokia 6.1 (Nokia 6 2018) ... 13,200/-
............................................................................................ and many more