Not sure why feval is used. Just using solve with ReturnConditions solves for the range of b that satisfies both inequalities.
syms b
a = 1;
eq1 = (1-sqrt(5))*a^2 < b;
eq2 = b < (1+sqrt(5))*a^2;
sol = solve([eq1;eq2],b,'ReturnConditions',true)
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