UPDATE 4-Libya says two Swiss businessmen must stand trial

* Two Swiss to be charged with tax evasion, other offences

* Men held since July 2008 after arrest of Gaddafi son

* Diplomatic row looks set to widen but no Swiss comment (adds quotes from Libyan Qouaim and wife of detained Swiss)

By Salah Sarrar and Stephanie Nebehay

TRIPOLI/GENEVA, Nov 12 (Reuters) - Two Swiss businessmen will be tried for tax evasion and visa irregularities, Libya said on Thursday, in a case that has caused a diplomatic row and opened the Swiss government to strong domestic criticism.

The men have been held in Libya since July 2008 following the arrest in Geneva of a son of Libyan leader Muammar Gaddafi on charges, later dropped, of mistreating 2 domestic employees.

The duo's detention has outraged many Swiss citizens and media, leading some to accuse Swiss President Hans-Rudolf Merz of mishandling the affair and demand his resignation.

An apology in August by Merz failed to secure a return home for Rachid Hamdani, who works for a construction company, and Max Goeldi, Libya head of the Swiss-Swedish electrical engineering conglomerate ABB (ABBN.VX).