Congestive Heart Failure (CHF): This is the most likely diagnosis due to the combination of bilateral leg swelling (edema) and shortness of breath, which are classic symptoms of CHF. The edema is caused by fluid buildup due to the heart's inability to pump efficiently, and the SOB is due to pulmonary congestion.

Chronic Kidney Disease (CKD) or Nephrotic Syndrome: These conditions can lead to fluid retention, causing edema, and can also affect the lungs, leading to shortness of breath.
Liver Cirrhosis: Cirrhosis can cause fluid buildup in the body, leading to edema, and can also lead to hepatopulmonary syndrome, causing shortness of breath.
Deep Vein Thrombosis (DVT): While DVT typically causes unilateral leg swelling, it can be bilateral and, if large, can lead to pulmonary embolism, causing shortness of breath.

Pulmonary Embolism (PE): Although it might not be the first consideration for bilateral leg swelling, a large or multiple PEs can cause significant shortness of breath and can be life-threatening if missed.
Superior Vena Cava Syndrome: This condition, often caused by a tumor or thrombosis, can lead to swelling in the legs and shortness of breath due to obstruction of venous return to the heart.
Cardiac Tamponade: Fluid accumulation in the pericardial sac can compress the heart, leading to impaired filling and subsequent shortness of breath and potentially leg swelling due to decreased cardiac output.

Lymphoma or Other Malignancies: Certain cancers can cause both leg swelling (due to lymphatic obstruction) and shortness of breath (due to pulmonary involvement or effusions).
Sarcoidosis: This autoimmune disease can affect the lungs and lymph nodes, potentially causing shortness of breath and leg swelling due to granulomatous infiltration.
Idiopathic Edema: A rare condition characterized by recurrent episodes of edema without an identifiable cause, which could potentially lead to shortness of breath if severe.