The invention claimed is:1. An apparatus for performing a communication service by using a plurality of modems, comprising:a first modem;a second modem; andan arbitration apparatus;wherein the first modem and the second modem multiplex a radio-frequency integrated circuit (RFIC);wherein the first modem is configured to perform a first communication service in a first frequency band;wherein the second modem is configured to perform a second communication service in a second frequency band;wherein an operating mode of each of the first modem and the second modem is switchable between a dual subscriber identification module (SIM) dual standby (DSDS) mode and dual SIM dual active (DSDA) mode depending on whether the first frequency band conflicts with the second frequency band.2. The apparatus according to claim 1, whereinthe arbitration apparatus, configured to determine whether the first frequency band conflicts with the second frequency band.3. The apparatus according to claim 2, wherein the arbitration apparatus is further configured to:when it is determined that the first frequency band conflicts with the second frequency band, control the first modem and the second modem to switch to the DSDS mode.4. The apparatus according to claim 2, wherein the arbitration apparatus is further configured to:when it is determined that the first frequency band does not conflict with the second frequency band, control each of the first modem and the second modem to switch to the DSDA mode.5. The apparatus according to claim 2, further comprises:a memory, wherein the memory is to store a correspondence table used to indicate a frequency band conflict status between the first modem and the second modem; andwherein the arbitration apparatus is configured to determine whether the first frequency band conflicts with the second frequency band according to the correspondence table.6. The apparatus according to claim 5, wherein the frequency band conflict status comprises:at least one of conflict frequency band information or non-conflict frequency band information of the first modem and the second modem.7. The apparatus according to claim 1, wherein the first modem and the second modem in the DSDA mode use their respective radio frequency devices, wherein each radio frequency device comprises at least one of the RFIC, a radio frequency front end (RFFE), or an antenna.8. The apparatus according to claim 7, wherein one of the first modem and the second modem uses a millimeter-wave radio frequency device, and the other modem uses a non-millimeter-wave radio frequency device.9. The apparatus according to claim 1, wherein the first modem and the second modem in the DSDS mode use a same radio frequency device in a preemption manner, wherein each radio frequency device comprises at least one of the RFIC, a radio frequency front end (RFFE), or an antenna.10. The apparatus according to claim 1, wherein the first modem and the second modem are integrated in a chip with an application processor.