\[
\boxed{
 \left\{ {\it a11}\,{\frac {{\rm d}^{2}}{{\rm d}{t}^{2}}}x \left( t \right) +{\it b11}\,{\frac {\rm d}{{\rm d}t}}x \left( t \right) +{\it c11}\,x \left( t \right) +{\it a12}\,{\frac {{\rm d}^{2}}{{\rm d}{t}^{2}}}y \left( t \right) +{\it b12}\,{\frac {\rm d}{{\rm d}t}}y \left( t \right) +{\it c12}\,y \left( t \right) =0,{\it a21}\,{\frac {{\rm d}^{2}}{{\rm d}{t}^{2}}}x \left( t \right) +{\it b21}\,{\frac {\rm d}{{\rm d}t}}x \left( t \right) +{\it c21}\,x \left( t \right) +{\it a22}\,{\frac {{\rm d}^{2}}{{\rm d}{t}^{2}}}y \left( t \right) +{\it b22}\,{\frac {\rm d}{{\rm d}t}}y \left( t \right) +{\it c22}\,y \left( t \right) =0 \right\} }
\]