The simple answer is yes, you can send multiple MIDI messages at once.
The actual MIDI communication is done at 480 Mbit using a complicated protocol which communicates in packets. A single packet can hold up to 128 MIDI messages, so unless you are transmitting a pretty incredible number of messages very quickly, typically the messages you transmitted at once will all end packed into the same packet. Even though the individual bits are sent 1 at a time, the host controller in your PC receives the entire packet into a memory buffer and verifies its CRC check before passing the data on to any MIDI software. The end result is all the MIDI messages within 1 packet are indeed received at once by the software on your PC. Or at least the operating system & drivers make them all available to the software at the same instant.
Even if multiple USB packets are used, the timing is incredibly fast relative to the pace of musical events. So even though a large burst MIDI messages spanning multiple USB packets aren't technically received simultaneously, the delay is so short that it's not worth any worry.
Something you might consider on the Teensy side is the usbMIDI.send_now() function. If you have sent less than 128 messages, this function tells Teensy's USB code not to wait for any more, to transmit a USB packet as soon as possible with only the messages it currently has buffered. If you care deeply about performance, this can reduce latency by up to 125 microseconds. But it does use the USB bandwidth less efficiently, so only call usbMIDI.send_now() after you've completed a group of messages and you do not expect to send any more immediately.