When is the Jacobian of a hyperelliptic curve 
$$y^2=x(x-1)(x-a)(x-b)(x-c)$$
a product of two elliptic curves? 
(This is a sort of reverse to
http://mathoverflow.net/questions/35060/when-is-a-product-of-elliptic-curves-isogenous-to-the-jacobian-of-a-hyperellipti). Obviously, it implies some algebraic relations between $a,b,c$; the question is: which ones?

P.S. This must be classical, but I am having some trouble figuring it out or finding it in the literature. There is an old example due to Jacobi, but is it all?

EDIT. As pointed out by abx, the set of parameters $(a,b,c)$  for which the Jacobian is isogenous  to a product is actually dense (e.g. in $\mathbb{C}^3$ with complex topology). For this reason, the question in this form does not have a reasonable answer. (In hindsight, it is obvious, but I missed it.)