Hi Leto -
If Noto Sans and Roboto are both licensed under the OFL then you can modify or combine them in any way as long as you retain the license and copyright notices from both fonts in the new one you make. See the OFL-FAQ starting with question 3.1:
Question: 3.1 Can I change the fonts? Are there any limitations to what things I can and cannot change?
Answer: You are allowed to change anything, as long as such changes do not violate the terms of the license. In other words, you are not allowed to remove the copyright statement(s) from the font, but you could put additional information into it that covers your contribution. See the placeholders in the OFL header template for recommendations on where to add your own statements. (Remember that, when authors have reserved names via the RFN mechanism, you need to change the internal names of the font to your own font name when making your modified version even if it is just a small change.)
Question: 3.2 I have a font that needs a few extra glyphs - can I take them from an OFL licensed font and copy them into mine?
Answer: Yes, but if you distribute that font to others it must be under the OFL, and include the information mentioned in condition 2 of the license.
You can also include the fonts in software that you sell, but the fonts themselves have to be distributed under the OFL. See OFL-FAQ starting with question 1.21:
Question: 1.21 What about including OFL fonts by default in my firmware or dedicated operating system?
Answer: Many such systems are restricted and turned into appliances so that users cannot study or modify them. Using open fonts to increase quality and language coverage is a great idea, but you need to be aware that if there is a way for users to extract fonts you cannot legally prevent them from doing that. The fonts themselves, including any changes you make to them, must be distributed under the OFL even if your firmware has a more restrictive license. If you do transform the fonts and change their formats when you include them in your firmware you must respect any names reserved by the font authors via the RFN mechanism and pick your own font name. Alternatively if you directly add a font under the OFL to the font folder of your firmware without modifying or optimizing it you are simply bundling the font like with any other software collection, and do not need to make any further changes.