How Can I Overlap 2 Different Signal

What kind of signals come from the HRMs? Please supply some details.

ECG signals

It is definitely possible to compare two different signals. The speed of the signals from a HRM are so slow that I doubt you actually need this comparison to be truly at the same time. Faster than a person can compare these signals will probably be good enough.

I see a few difficulties in this concept.

A golden standard from any data analysis is almost never actually found in the real world. The standard is an average or other smoothing of all of the measurements from a set of data. If what deviates these real signals from the average is a purely random source then 68% of the time these deviations will be less than one σ from the average. Where does one put the threshold for detecting a trip condition?

The amount of data storage and processing power needed to achieve this comparison might make the the cost versus benefit ratio too expensive.

Then again, these concerns and questions are precisely what good research is supposed to identify.

Is there any way to detect 2 different signals(each from one of the HRM) on the same time domain.

You could use a dual channel digital oscilloscope. If you don't have one available, you can probably rent one.

You can purchase a $29 datalogger with four voltage inputs +10/-10 V allowed, and sufficient bandwidth for millisecond work at the highest sampling rates. I do not think you could resolve one microsecond, but your application has no such requirement.

Goopgle it. I have two of them in my backyard shed/workshop, and they work great.

You can set up your own engineering units on them, etc.

You can also do the same with a basic Arduino board, if you know how to oversample and average to get more resolution. At least I think that is how it is done. I know I have seen data with way more digits than what a 10 bit word would allow for.

with digital data acquisition to provide a record.

Test with two identical ecg units in each channel before & after the comparison, to validate the test,

Test with a known test pattern of similar voltage & frequency, maybe sawtooth vs. squarewave, to prove that the differential values produced by the analytical calculation between the two samples is also valid.

Mathematically analyze the test sawtooth vs test squarewave patterns, and comparing those results to the test pattern differential calculated from the numerical data acquired, using the identical calculation to the ecg.

From this you should be able to determine what the accuracy is for your desired resolution / sample period.

Something like this?

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4273439/

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4273439/figure/Fig1/

This is pretty cool...

http://www.theheartcheck.com/

Don't know how comfortable this would be....but a lot of features including Bluetooth...

https://www.getqardio.com/en/qardiocore-wearable-ecg-ekg-monitor-iphone/

Looks like the competition already beat OP to the market a long time ago?

Accuracy of the Digital readings of different HRM varies depending on design criteria used by each chosen manufacturers! And there are hundreds of them!

To validate, you may need to be familiar with those manufacturer(s) product line as a starting point... Once familiar, your validation needs may no longer exist?

A/D conversion, sampling, and filtering techniques are examples of their key differences used in their respective products..

You also need familiarity on the physiology of your subject where the signals are goung to come from? Else, you may also cheat by using some readily available patient simulators?