how to create two PWM with 15 kHz difference

you should have attached the datasheet of LPC1768 also.

okay so i went through the datasheet. here is the link(https://www.nxp.com/docs/en/data-sheet/LPC1769_68_67_66_65_64_63.pdf)
and it is mentioned that LPC1768 has 2 PWM Units where one unit is 6 general purose PWM and second unit is Motor Control PWN.
You can easily program these two PWM to your specifications for your PWM requirement.
Also the datasheet states that The LPC1768/67/66/65/64/63 operate at CPU frequencies of up to 100 MHz. 
So you can generate the PWM frequency mentioned about your project. 
You can use the PWM.c library to configure the PWM fucntions. 
Also i would recommend to read datasheet properly. they mention most of the things.  

In data sheet, there is mentioned that there are 2 PWM units of micro-controller device LPC1768 b32 bit arm cortex M3. It is easy to program these two PWM units to as per the specifications for our PWM signal requirement. The micro-controller you are using operates at 100 MHz of the CPU frequency. According to which you can generate your required signals. It can be done by using some specified libraries for your controller. Here I would like to recommend you to contact SSLA for getting better assistance for your project.

If you check the data sheet, there is mentioned that there are 2 PWM units of micro-controller device LPC1768 b32 bit arm cortex M3. It is easy to program these two PWM units to as per the specifications for our PWM signal requirement. The micro-controller you are using operates at 100 MHz of the CPU frequency. According to which you can generate your required signals. It can be done by using some specified libraries for your controller. Here I would like to recommend you to contact Sierra Software Ltd. for getting better assistance for your project.