Is it possible to use USB and CAN on STM32F1 devices at the same time?
No, it isn't.
In low, medium-, high- and XL-density devices, the USB and CAN share a dedicated 512-byte SRAM memory for data transmission and reception, and so they cannot be used concurrently: the shared SRAM is accessed through CAN and USB exclusively.
So the USB and CAN can be used in the same application but not at the same time on STM32F1.
However, for other (or newer) STM32 families, it is possible to use USB and CAN peripherals concurrently.