|There is no guaranteed safe way to do this.|
I'm not sure if any of the partitioning tools out there have the facility to merge partitions.
Your first step should be to back up everything you want to keep on both partitions.
Is one of these your System Drive C? Does the laptop have a hidden "recovery" partition?
As I understand the use of these type partitioning tools it would be possible to delete the unwanted partition and then expand the remaining partition into the unpartitioned space. I'm not sure what happens to the file system (Fat32 or NTFS) when a partition is expanded into unpartitioned space. It may be necessary to re-format the new large partition in which case you lose your operating system.
My recommendation. Live with your original decision to split the drive. Unless you have left your C drive too small to accommodate the operating system.
Goin' Fishin' (Some day)