Director wants to buy retiring directors share

Hello allMaybe a basic question to some, but would be grateful for some advice. Valuing a business - Director A wants to retire, Director B wants to buy A's shares etc. and continue business. Both 50% shareholders'. No shareholder's agreement, yes I know unfortunate. Is the first stage commercial valuer etc.? Will I need a second opinion, as accountant has a conflict of interest by representing both directors? I am the wife of Director B. Two companies involved, holding company (has all the assets, property (owned outright), equipment,vehicles etc. , and trading company. Without sounding pretty naive, but, as the two directors are at loggerheads, even over one has had £1 more than the other, where do I start? Any comments greatly appreciated.