One sensor has a baud rate of 115200, the other 9600. I think this
will create problems in the serial because I can only choose one baud
Why do you think the sensor has a baudrate?
Both the sensors seem to use the "serial connection" of the Arduino to log their information.
What do you think? The first sensor is the pulse sensor, while the
other is the gsr sensor by grove. Link: http://pulsesensor.com/ Link
I think it'll work fine.
Looking at the facts of your sensors, the pulse sensor does have some kind of weird low-level timing/ADC access. But your Grove GSR sensor can basically be read out through an analog input.
So what to choose?
You could choose 9,6k or 115k2 (9600 or 115200). It wouldn't make all that much difference. 9600 or 115200 will work fine. 115200 is a lot faster, but for debugging purpose 9600 is used, since it's often default and (often) more reliable.
I don't think there is an issue with the two serials. You can easily log both messages to the same serial output.
Try using 115200 since timing is essential in your case, you want your sensor to be response/fast.
They do both use the same analog to digital converter on your board. An Arduino can't check both sensors at the same time. But by switching between them very fast, you will get a resolution that's good enough.